In general, various kinds of processes, such as a film forming process, an etching process, a thermal process, a modifying process and a crystallizing process, are repeatedly performed to a process object such as a semiconductor wafer in order to fabricate a semiconductor integrated circuit. When each of the various kinds of processes is performed, process gases required for the process are supplied into a processing vessel. JP 10-321613A discloses one example of an apparatus that performs a film forming process out of the foregoing processes. This film forming apparatus has a shower head structure disposed on a ceiling portion of a processing vessel which can be evacuated. A raw gas and a support gas such as an oxidizing gas or a reducing gas are supplied into the processing vessel through gas injection holes formed in the shower head structure, thereby depositing a thin film on the heated semiconductor wafer by CVD.
When a raw gas having a relatively low vapor pressure and a high activation energy is used, a film forming reaction occurs as soon as the raw gas is mixed with a support gas (e.g., oxidizing gas) before the raw gas is injected from the shower head structure. In order to prevent this, there is employed an injection system in which the raw gas is brought into contact with the support gas only after the raw gas is injected into the processing vessel from the shower head structure. Such an injection system is called a post-mix system.
FIG. 7 shows one example of a film forming apparatus employing the post-mix system. A film forming apparatus 2 includes a cylindrical processing vessel 4 which can be evacuated. A mount table 6, allowing a semiconductor wafer W to be placed thereon, is arranged in the processing vessel 4. A heater 8 is embedded in the mount table 8. A shower head structure 10 is arranged on the ceiling portion of the processing vessel 4 to supply a film forming gas into the processing vessel 4. The shower head structure 10 is composed of a plurality of head plates 10A to 10D stacked in layers and connected integrally with each other via bolts 12 (only some of them are shown in FIG. 7).
Plural recesses and gas channels connecting the recesses to each other are formed in the surface of each of the head plates 10A to 10D. Plural gas diffusion chambers 14A, 14B and 14C are formed when the head plates 10A to 10D are assembled. In the illustrated example, the gas diffusion chambers 14A and 14C are communicated with each other. The lowermost head plate 10A serves as a gas injecting plate provided therein with a number of gas injection holes 16. The gas injection holes 16 are classified into: a first group of gas injection holes 16A that communicate with the gas diffusion chamber 14A to inject the oxidizing gas such as O2 gas; and a second group of gas injection holes 16B that communicate with the gas diffusion chamber 14B to inject the raw gas. The raw gas and the oxidizing gas flow separately through the shower head structure 10 without being mixed with each other, are individually injected into a processing space S through the gas injecting holes 16A and 16B, respectively, and then for the first time are mixed with each other in the processing space S. Consequently, deposition of an unnecessary film inside the shower head, which may result in particle generation, can be prevented, while a necessary film can be deposited substantially only on the wafer.
A cooling mechanism 18 such as a cooling jacket is attached to the peripheral portion of the upper surface of the shower head structure 10. The cooling mechanism 18 cools the lowermost head plate 10A down to a predetermined temperature in order to prevent the raw gas, which is likely to be thermally decomposed, from being decomposed immediately after the raw gas is injected from the gas injecting hole 16B, thereby to prevent adhesion of an unnecessary film, which may result in particle generation, to the lower surface of the head plate 10A, or the gas injecting surface.
Since the shower head structure 10 is constituted by stacking the plural flat head plates in layers and by joining them via the bolts, and moreover, the interior of the processing vessel 4 is maintained at a relatively low pressure, thermal conductivity among the head plates 10A to 10C is not so high. Therefore, the cooling mechanism 18 fixed to the head plate 10C can not effectively control the cooling of a part, near the gas injecting surface, of the head plate 10A. As a result, an unnecessary film may possibly adhere to the gas injecting surface.
As a film forming process to a wafer is performed repetitively, unnecessary thin films may possibly be deposited on the gas injecting surface, facing the processing space, in a region of several millimeters to several centimeters in diameter around each gas injection hole for injecting the raw gas. Such unnecessary thin films will peel off to be in particles, if they are left unattended. Thus, the shower head must be periodically cleaned. In the illustrated apparatus, since the shower head structure 10 is constituted by integrating the plurality of head plates 10A to 10C via the bolts 12, when the lowermost head plate 10A which is the main object to be cleaned is removed, the substantially whole shower head structure will be broken into individual component parts. Thus, the maintenance work is very complicated and time-consuming.